NQR testing is used for detecting the presence or disposition of specific substances. It depends on the energy levels of quadrupolar nuclei, which have a spin quantum number I greater than 1/2, of which .sup.14 N is an example (I=1). .sup.14 N nuclei are present in a wide range of substances, including animal tissue, bone, food stuffs, explosives and drugs. One particular use of the technique of the present invention is in the detection of the presence of substances such as explosives or drugs. The detection may be of baggage at airports, or of explosives or drugs concealed on the person or buried underground.
In the molecular environment of compounds in crystals, the nature and disposition of the electrons and other atomic nuclei close to the nucleus of interest produce an electric field gradient at the latter which interacts with its electric quadrupole moment to generate a set of energy levels, the transition frequencies between which are characteristic for a given substance. The presence of this frequency or frequencies not only indicates which nuclei are present, but also their chemical environment, thus indicating specific substances or types of substances in any tested sample.
In conventional NQR testing a sample is placed within or near to a radio-frequency (r.f.) coil and is irradiated with pulses or sequences of pulses of electro-magnetic radiation having a frequency which is at or very close to a resonance frequency of the quadrupolar nuclei in a substance which is to be detected. If the substance is present, the irradiant energy will generate a precessing magnetization which can induce voltage signals in a coil surrounding the sample at the resonance frequency or frequencies and which can hence be detected as a free induction decay (f.i.d.) during a decay period after each pulse or as an echo after two or more pulses. These signals decay at a rate which depends on the time constants T.sub.2 * for the f.i.d., T.sub.2 and T.sub.2e for the echo amplitude as a function of pulse separation, and T.sub.1 for the recovery of the original signal after the conclusion of the pulse or pulse sequence.
In conventional NQR testing, either a substantial part of the f.i.d. is measured after each pulse or the responses are measured as echoes in the relatively short sampling periods between or following a succession of two or more pulses. Usually the results from a number of test pulses or test sequences are accumulated to improve the signal-to-noise ratio. Various schemes of pulse sequences have been used.
The present invention arises, in one aspect, from the surprising discovery that, in the detection of the presence of a particular substance in a given sample using nuclear resonance techniques, interfering signals may arise from the sample which may swamp the genuine nuclear resonance signals. This is particularly so in circumstances where the amount of the particular substance is much less than the amount of the remainder of the sample, and the interfering signals arise from the remainder of the sample. Such circumstances, it has now been discovered, occur commonly. For example, small amounts (maybe only a few tens of grams) of narcotics are frequently concealed within bulky pieces of airport baggage. Many common household items carried in baggage, it has been found, are likely to give rise to interfering signals.
The problem of one kind of interference when testing a particular substance is known from a book entitled "Experimental pulse NMR", by Fukushima, E. et al, Addison-Wesley Publishing Company, Inc., pp. 466ff. The interference takes the form of spurious ringing caused by piezo-electric resonance of the substance. However this problem is not disclosed in the context of detecting the presence of the substance within a given sample.
It is known from this book to solve this particular interference problem by using an electrostatic shield (Faraday shield) between the sample and the NMR probe (usually a coil) to reduce the interference. However, such a shield can be bulky and unwieldy. Furthermore, it has been discovered pursuant to the present invention that, particularly with NQR testing, there may be different types of interference other than that caused by piezo-electric resonance, and that the Faraday shield may not successfully reduce all these types of interference.
Also, it is known from a book by Cady, E. B. entitled "Clinical Magnetic Resonance Spectroscopy", Plenum Press (1990) (see pages 160ff.) in the field of Nuclear Magnetic Resonance Imaging to adjust the NMR probe according to the nature (for instance, size) of the body being imaged. This adjustment is not to take account of interfering signals from the body; rather, the body is actually the "substance" under test, and the adjustment is to ensure appropriate "matching" of the probe and the body. A similar technique is known from European Patent No. 0 180 121.
Such adjustment has not hitherto been contemplated for nuclear resonance techniques involving the detection of the presence of a particular substance in a given sample, partly, perhaps, because the amount of the substance under test is generally so small in relation to the total amount of the sample and the volume probed by the probe (that is, the filling factor is so small), that the possibility of requiring matching between the probe and the substance/sample has ever been contemplated. Typically, the filling factor might be only fraction of one percent.
According to a first aspect of the present invention, there is provided a method of detecting the presence of a particular substance in a given sample comprising exciting nuclear resonance in the substance and detecting the response signals from the substance, using a probe, and adjusting the probe in dependence on the character of the sample.
It has been found pursuant to the present invention that adjustment of the probe in dependence on the character of the sample can have the advantage of reducing interference from many types of substances (other than the substance of interest) commonly found in a typical sample such as airport baggage. Hence the invention can provide more accurate and sensitive tests than if such adjustment does not take place, and can also reduce the false alarm rate.
In the present invention, there is no need to adjust the probe in dependence on the character of the particular substance at all. The filling factor of the substance would be assumed to be so small (a fraction of one percent, say, corresponding to a few tens or hundreds of grams of explosive in a suitcase) that adjustment of the probe in dependence on the character of the substance is simply not necessary.
Preferably, the probe is adjusted (or adjustable) whilst it is in operation. This feature is particularly advantageous if baggage is being checked for the presence of explosives or drugs, for instance on a conveyor at an airport. The nature of the interference from each individual item of baggage may be different, and hence the sensitivity of the tests can be improved by adjusting the probe individually for each item of baggage.
The principles underlying this aspect of the invention are as follows. In the case of NQR testing, it has been determined pursuant to the present invention that there are two groups of materials which produce interference problems that may require adjustment of the r.f. probe usually used in NQR testing.
The first group includes metallic conductors which conduct electronically, such as brass, copper and aluminium. Such materials may be commonly found in many types of objects in baggage, for instance, in electronic circuitry. For this group, the interference effects are caused largely by eddy currents induced by the NQR r.f. excitation. These effects may be particularly acute in the case of metallic loops such as leads or connections on printed circuit boards or on metal sheets. They cause electrical loading and thereby alter the inductance of the sample coil forming part of the r.f. probe, and hence change the resonance frequency of the probe (since the probe may be considered to include a resonant circuit of which the inductance of the coil forms one part). If the quality factor (Q) of the coil lies within the usual range (20 to 60, for example), so that the probe has a relatively narrow bandwidth, then the resonance frequency of the probe may be shifted so far that the sensitivity for NQR detection is significantly reduced. Further, a change in the inductance of the sample coil may also adversely affect the matching between the probe and the r.f. transmitter. Also, the Q factor itself may be adversely affected by a change in the inductance of the sample coil, so that the sensitivity of detection is adversely affected. It will be understood that Q=.omega.L/R (where .omega. is the (angular) resonance frequency of the probe, L is the inductance of the coil, and R is the series resistance of the probe) and that signal-to-noise ratio (and hence sensitivity of detection) varies as the square root of Q.
Included in the first group as a special case are magnets and magnetic materials. Such magnets, if made of electronic conductors, as would usually be the case, would cause interference effects similar to those described in the preceding paragraph. However, they can also cause a shift in and possibly even a splitting of the particular resonance line of interest. Although the probe may require re-tuning to take account of these additional effects, it is not believed that this would normally be necessary.
The second group of materials includes non-metallic materials which conduct ionically, such as wet sand or soil and the electrolyte in batteries. Like the first group of materials, this second group also has the effect of altering the resonance frequency and Q factor of the probe, but this time through the mechanism of dielectric loss, which causes a change in the resistance of the probe and also causes a change in its inductance.
In putting the first aspect of the present invention into practice, the invention suitably provides that the presence (and character) of such interfering materials is detected and then acted upon by effecting suitable adjustment of the probe.
Detection of such materials is preferably effected by measuring the back-reflected r.f. power from the sample, for example, using a directional coupler. An increase in back-reflected power indicates that the probe is no longer correctly tuned and hence indicates the presence of an interfering material.
Adjustment of the probe may be effected firstly by adjusting the matching of the probe to the r.f. transmitter (that is, by adjusting the impedance of the probe). This may be achieved, for example, by insertion of a ferrite rod into a tapped inductance in the probe circuitry, the rod being moved by a stepper motor programmed to seek the position of minimum back-reflected wave as measured for instance by a directional coupler.
Secondly, the resonance frequency of the probe may be adjusted by automatically tuning the probe via a servo or stepper motor on the axis of a variable capacitor in the tuning circuitry. The capacitance is varied until the r.f. field amplitude (B .sub.r.f) is a maximum, as monitored by a simple pick-up coil placed near the r.f. coil. The alternating voltage induced in the pick-up coil could be rectified by either a diode or a quadrature detector and the resulting d.c. voltage fed to the control computer by means of an Analog to Digital Converter, where it could be compared with a known reference voltage, the difference between the two being used to control the stepper motor. The same quadrature detector and transient recorder could be used both for the main coil and the pick-up coil, which could render this technique relatively cheap.
Finally, it might be necessary to check that the Q factor of the r.f. coil was unchanged, which could be done automatically in a short time by frequency sweeping the r.f. pulse in small steps, so as to measure the frequency difference .DELTA.v=.vertline.v.sub.1 -v.sub.2.vertline., where v.sub.1 and v.sub.2 are the frequencies at which the r.f. current (as measured by a pickup coil) has dropped to 0.71 of its maximum value, and then using the well-known equation ##EQU1##
to derive a value for Q.
If the Q has diminished significantly its value can be restored by removing damping devices such as metal paddles or series resistors deliberately fitted for this purpose. Alternatively, the NQR test instrument, once forewarned, could be programmed to perform the test with more accumulations to overcome the effect of the drop in Q; since signal-to-noise ratio is proportional to Q.sup.1/2, the number of accumulations must be increased by the factor Q.sub.old /Q.sub.new. Further, the r.f. power needs to be increased if Q is lower in order to maintain the same flip angle.
It will be appreciated that not all of the above three adjustments may be necessary. However, if they are, they should preferably be carried out in the order (i) Tuning, (ii) Matching, (iii) Q factor adjustment, although other orders are possible. Further, the various adjustments may interact with one another, so that an interactive set of adjustments may need to be carried out. The three adjustments proposed above may take, say, between 0.1 and 1 second, perhaps 0.2 to 0.5 seconds.
In certain circumstances, for example, a sheet of RDX next to a metal sheet (or sandwiched between two metal sheets) of sufficient thickness, the r.f. field may be very significantly compensated by eddy currents, thereby considerably reducing sensitivity. In these and other circumstances the NQR testing apparatus may be arranged to provide an alarm signal of insensitivity or of the presence of metal. Appropriate remedial action (such as increasing the r.f. power and/or number of accumulations, or hand-inspection of the particular object under suspicion) can then be taken.
According to a second aspect of the present invention, there is provided a method of nuclear resonance testing a sample, comprising exciting nuclear resonance in the sample, and detecting and processing the response signal from the sample in such a way as to at least partially filter out interfering signals, not due to nuclear resonance, arising from the sample.
It will be understood that, in the context of the detection of the presence of a particular substance in a sample, it would usually not be the substance to be detected but the remainder of the sample which would give rise to the interfering signals.
By filtering interfering signals via the detecting and processing means rather than by means of a Faraday cage physically surrounding the sample, the present invention can be considerably simpler to put into practice, especially in the case of baggage being tested on a conveyor belt.
It is possible to, and in certain cases of strong electric interference, it may be necessary to, use both a Faraday cage and signal detection and processing techniques.
The invention is preferably performed in the absence of an applied magnetic field.
A further discovery pursuant to the present invention is that there are materials such as ferromagnetic materials, certain steels or nickel-plated objects (for instance, screws or key-rings) which produce a strong spurious signal immediately following the r.f. pulse which is frequency and phase coherent with the pulse and which cannot therefore be reduced by repeated accumulations (unlike most other interference, which can be). The precise cause of this type of interference has not been proven, but it is believed to emanate from ferromagnetic or like resonance effects in the B.sub.1 field of the sample coil. It should be emphasised that this interference is not an artefact of the particular detection apparatus used, but a feature of the material itself.
Piezo-electric materials may also produce frequency and phase coherent mechanical resonances which may lie in the frequency range of the NQR response. For instance, sand can produce interference at an r.f. frequency of 5 MHz.
It may be necessary to compensate for this interference by adjusting the probe, as discussed above, unless the interference emanates from piezo-electric materials, which are non-conductors. Further, since the effects of the interference cannot be reduced by repeated accumulations, there is a requirement to provide suitable techniques for reducing these effects. The present invention provides these techniques in two further aspects.
In a first such aspect, there is provided a method of nuclear resonance testing a sample, especially one which gives rise to interfering signals, not due to nuclear resonance, comprising exciting nuclear resonance in the sample, and detecting the response signal from the sample, there being a delay of a predetermined duration between the end of the excitation and the beginning of the detection.
This aspect arises from the discovery pursuant to the present invention that the spurious signals constituting the interference decay rapidly after the r.f. pulse, usually within, say, 350, 500, 750 or 1000 .mu.s of the end of the r.f. pulses. Thus, provided the decay time of these signals is significantly less than the f.i.d. time (T.sub.2 *) or echo decay times (T.sub.2, T.sub.2e) of the NQR response, useful response data can be derived by delaying detection of the response signal for a predetermined duration, until all or the bulk of the spurious signal has decayed. A delay of 500, 600 or 700 .mu.s has been found empirically to be satisfactory in most circumstances. A sufficient decay for the response signal might be to below 20, 10 or 5% of its initial peak value.
On the basis of a delay of 500, 600 or 700 .mu.s, this aspect of the invention would be particularly suited to materials exhibiting a T.sub.2 *, T.sub.2 or T.sub.2e greater than, say, 1 ms, so that the spurious signal has ample time to decay before there is a serious loss in the NQR response signal. One such material is the explosive RDX, which has one frequency with a T.sub.2 * of 1.4 ms at room temperature.
Thus it will be understood that in practice there are both upper and lower bounds on the duration of the delay. The lower bound is dictated by the requirement that the spurious interfering signals have sufficient time to decay to negligible or near-negligible levels. The upper bound is dictated by the requirement that the NQR response signal has not decayed so much that no useful response data can be detected.
In one preferred embodiment, the excitation is such as to cause a free induction decay response signal, and the delay occurs during this response signal. In another preferred embodiment, the excitation is such as to cause an echo response signal, and the delay occurs between the end of the excitation (normally the refocussing pulse) and the echo response signal.
In a second such aspect of the present invention, there is provided a method of nuclear resonance testing a sample, comprising exciting nuclear resonance in the sample using at least two different types of excitation, such that the nuclear resonance can be distinguished from interfering signals, not due to nuclear resonance, arising from the sample, from a comparison between the response signals from the different types of excitation; detecting the response signals from the sample; and comparing the detected response signals from the different types of excitation.
This aspect arises from the discovery pursuant to the present invention of certain ways in which the spurious interfering signals can be distinguished from the genuine nuclear resonance.
In one preferred embodiment, this distinction can be made by arranging that the different types of excitation are such as to produce, for the respective response signal from each different type of excitation, a nuclear response signal and an interfering signal of a different relative phase. For example, if the relative phase is 180.degree. (anti-phase), the different types of excitation may be such as to produce nuclear response signals of different relative phase.
In another preferred embodiment, this distinction can be made by arranging that the different types of excitation are such as to produce, for the respective response signal from each different type of excitation, a nuclear response signal and an interfering signal of a different relative amplitude. Preferably, the amplitude of the nuclear response signal differs according to the type of excitation, whilst the amplitude of the interfering signal remains the same. For example, the different types of excitation may be such as to saturate the nuclear response to different extents.
The invention in its various aspects extends to apparatus equivalent to the above methods. Apparatus features analogous to the various method features can be provided within the scope of this invention. Further, the various aspects and features of the invention may be combined in any appropriate way.
For example, there is provided according to the present invention apparatus for nuclear resonance testing a sample, comprising means for exciting nuclear resonance in the sample using at least two different types of excitation, means for detecting the response signals from the sample, and means for comparing the detected response signals from the different types of excitation.